A gas turbine engine combustor usually comprises a combustion chamber into which fuel is introduced at its upstream end through a fuel injection nozzle or nozzles. Air is introduced both at the upstream end and throughout the combustion chamber length. The air so introduced serves two purposes: it supports the combustion process which takes place within the chamber and it provides cooling of the chamber.
One of the ways in which air is introduced into the combustion chamber for cooling purposes is through holes located in the combustion chamber walls and also sometimes in a heat shield usually located at the upstream end of the chamber and surrounding the fuel injection nozzle. A combustion chamber cooled in this way is described in GB 2221979A. The holes are so arranged that those in the heat shield direct the air passing through them towards the fuel nozzle. Those holes in the combustion chamber walls are arranged so as to direct the air passing through them in a generally downstream direction. In both cases, the air forms a film on the internal surfaces of the walls, thereby ensuring that the walls do not overheat.
It is also known from NASA Technical Note NASA TN D-8248 "Streakline Flow Visualization of Discrete-Hole Film Cooling with Normal, Slanted and Compound Angle Injection" Raymond S. Colladay and Louis M. Russell, Lewis Research Center, Cleveland, Ohio 44135, Sep. 1976 to inject a flow of cooling air through slanted holes in a wall at an angle of 45.degree. laterally to the main gas flow across the wall.
It has been found with this sort of arrangement for the introduction of air into the chamber that cooling is not as effective as is normally desirable for a given flow of air. Additionally carbon deposition can take place and it is sometimes difficult to ensure that harmful emissions from the chamber, that is those of carbon monoxide, unburned hydrocarbons, smoke and the oxides of nitrogen, are below statutory limits. These emissions tend to accumulate in the cooling air film and are swept out of the chamber in the film before they have chance to be consumed by the combustion process. A further problem is that in order to ensure effective cooling, relatively large amounts of air are required. This means that the amount of air for primarily taking part in the combustion process is limited, thereby giving rise to less than efficient combustion.